How can I hook into the wp_mail function used by BackWPup?

Question

I know this question is partially concerns a third-party plugin, BackWPup, which is off-topic, but in general, in terms of WordPress and PHP: is it possible to hook into the wp_mail function so can I append text to the plugin's PHP generated email?

The email is sent by BackWPUp, a WordPress database backup plugin, and the email contains a success or fail message about the backup file. In my case, the email will go to a general email support address, and may be received by someone who may not fully familiar with how to deal with the email. I need to be able to add more instructional and explanatory text to the email, preferably at the beginning of the email.

And I need to add that text by using an external php function, because if I modify the plugin, the changes will be overwritten when the plugin is updated; so I need to add the function to either in the theme's functions.php file, or make my own simple WordPress plugin.

The wp_mail function in the plugin that is used appears to be:

wp_mail(
$this->job['mailaddresslog'],
$subject,
file_get_contents( $this->logfile ),
$headers
);

is it possible to hook into the $messageand add additional text? It has been replaced with file_get_contents in the function. Do I need to hook into file_get_contents?

https://developer.wordpress.org/reference/functions/wp_mail/

Edit 7/28/19

I added a check for the subject line, as all emails sent had the additional text appended. Now, only BackWPUp emails get the additional text.

add_filter( 'wp_mail', 'wpse343761_filter_message' );
function wpse343761_filter_message( $atts ) {

    if ( stripos( $atts['subject'], 'BackWPup')) {

    if ( ! empty( $atts['message'] ) ) {
        $atts['message'] .= 'Your additional text';
    }
}
    return $atts;
}

New questions on modifying message: how can I use strip_tags on the message to convert the default html email to non-html email? Adding strip_tags($atts['message']); before the return doesn't do anything.

And, how can the "additional text message" be added to the beginning of the email? It's added to the end right now.

Answer 1

Looking at the source of the wp_mail() function, there's a filter right near the top:

$atts = apply_filters( 
    'wp_mail', 
    compact( 'to', 'subject', 'message', 'headers', 'attachments' )
);
// Expanded for clarity.

(Aside: compact() is a PHP function that creates an array from a set of arguments. In this case, it's making an array of the $to, $subject, $message, $headers, and $attachments parameters passed to wp_mail().)

...and then a little further on:

if ( isset( $atts['subject'] ) ) {
    $subject = $atts['subject'];
}

So yes, you can filter the message. Something like this should do the trick:

add_filter( 'wp_mail', 'wpse343761_filter_message' );
function wpse343761_filter_message( $atts ) {
    if ( ! empty( $atts['message'] ) ) {
        // If there's already a message, add to it.
        $atts['message'] .= 'Your additional text';
    } else {
        // If there's not a message set yet, set one.
        $atts['message'] = 'Your desired message';
    }
    return $atts;
}

Note: This code is untested.

Edit: In response to your update And, how can the "additional text message" be added to the beginning of the email?:

Change this line:

$atts['message'] .= 'Your additional text';

to:

$atts['message'] = 'Your additional text' . $atts['message'];

And to your strip_tags() usage question, you can do this:

$atts['message'] = strip_tags( $atts['message'] );

on the line before you return. (You can't just use strip_tags( $atts['message'] ) because that doesn't assign the stripped string to anything.)